The ADVENT Shieldbearer, arguably the ultimate support unit in XCOM2, is a defensive powerhouse. Shieldbearers have special armor, as you can see in the images here, and can provide a Shield Bonus to other nearby enemy units on the battlefield.

So, imagine the toughest threats the aliens have at their disposal, then imagine those threats with boosted defense - it’s not an ideal scenario, Commander. Upon meeting an ADVENT Shieldbearer, Central advises you focus all efforts to dispatch this unit as quickly as possible.